Bubble extensions from devcontainer to local workspace recommendation (#7)

- It _does_ cause `github.dev` (i.e. where you go when you press
  <kbd>.</kbd> on your repo) to prompt for install of YAML extension,
  but not PDF (probs not on an allowlist)

- These /may/ also make the devcontainer.json ones redundant, I'm not sure
  (devcontainers were being broadly tempremental at me but I thought a
  log line suggested such a thing)

Author: donaldguy

.vscode/extensions.json

@@ -0,0 +1,6 @@
+{
+    "recommendations": [
+        "mathematic.vscode-pdf",
+        "redhat.vscode-yaml"
+    ]
+}